Binocular fixation on a near object requires a precisely controlled inward movement of the two eyes. This motor response, called ocular vergence, is driven by two stimuli: disparity, or non-correspondence between the two retinal images, and retinal blur. When isolated each stimuli will produce both lens and vergence motor responses. Vergence due to blur stimulus alone is called accommodative vergence and lens response due to disparity stimulus alone is called vergence accommodation. During normal binocular vision the two components are combined to produce the coordinated motor response essential to clear single vision. Disruption of the balance between these interactive response components underlies a majority of clinical observed oculomotor disorders. Early theories emphasized the dominance of the blur-driven component (accommodative vergence) in the oculomotor response. However, recent experimental results refute these long standing theories and suggest a more complex interaction between blur and disparity driven components during binocular fixation. In particular, the frequently ignored disparity-driven component (vergence accommodation) has been shown to influence both vergence and lens binocular responses. Initial experiments will quantify the relationship between vergence accommodation and sustained vergence error (fixation disparity) in subjects with normal binocular motor responses, and also establish a nominal range for the magnitude (effective strength) of this and other binocular vergence components. A clinical segment will then attempt to isolate oculomotor disorders specifically related to abnormal levels of vergence accommodation. Finally, the influence of orthoptics training on vergence accommodation and other components related to binocular coordination will be investigated.